The invention provides a power storage device capable of preventing reduced energy efficiency of the power storage device and of avoiding variations in temperature distribution. The power storage device includes a positive electrode and a negative electrode, and a solid electrolyte layer placed between the positive electrode and the negative electrode and including a group of particles, wherein the density of particles in a first area of the solid electrolyte layer is lower than the density of particles in a second area which has higher heat radiation than the first area.